1. Field of the Invention
This invention concerns an improved electrode having a porous nickel surface, a process for making it and its use as a cathode in a brine electrolysis cell. In particular, the invention relates to an electrode made by a process which includes interdiffusing aluminum and nickel to form a nickel-aluminum alloy layer and subsequently selectively dissolving aluminum from the thusly formed layer.
2. Prior Art
It is known that an active, porous nickel can be produced by selectively dissolving a soluble component, such as aluminum or zinc, from an alloy of nickel and the soluble component. Such porous nickel and the alloy from which it is produced are frequently referred to, respectively as "Raney nickel" and "Raney alloy", named after their inventor. Several specific methods for producing Raney nickel and a variety of uses for this material have been suggested in the past.
W. Vielstich, Chemie-Ingenieur-Technik 33, p. 75-79 (1961) describes a double-skeleton electrode of Raney nickel prepared by mixing powdered Raney alloy (e.g., nickel and an alloying component such as aluminum) with a skeleton material of pure metal powder (e.g., carbonyl nickel), pressing, sintering and then dissolving the alloying component from the Raney alloy. The surface layer of such an electrode comprises a dispersion of active Raney nickel particles embedded in a skeleton of inactive solid nickel particles. The electrode is suggested for use as, among other things, hydrogen-evolution cathode in a chlorine-alkali electrolysis cell of the diaphragm type. However, double-skeleton electrodes, prepared by powder metallurgy techniques, have low mechanical strength and are generally unsuited for forming into large screen electrodes as are desired for commercial brine electrolysis processes.
A method for preparing Raney Nickel sheet material by spraying molten particles of Raney alloy (e.g., nickel and aluminum alloy) on a metallic substrate and then selectively dissolving the aluminum is disclosed in Goldberger, U.S. Pat. No. 3,637,437. This material is suggested for use as catalytic cathodes for fuel cells. However, cathodes prepared by the disclosed technique generally have surfaces of low porosity and a tendency to spall.
Hahndorf et al., U.S. Pat. No. 3,272,728 and Hooker Chemicals & Plastics Corp. German Offenlegunsschrift No. 2,527,386 (based on F. Hine, U.S. Ser. No. 489,284) disclose electrodes having Raney nickel surfaces that are prepared by electrolytically codepositing nickel and zinc from an inorganic electrolytic bath onto a metal substrate to form a Raney alloy from which zinc is then selectively dissolved. The electrodes are reported to possess low hydrogen overvoltage characteristics. However, the disclosed techniques generally produce very thin layers of the Raney alloy and of the subsequently formed Raney nickel.
British patent specification No. 1,289,751 (Siemens Aktiengesellschaft) discloses a method for making porous nickel electrodes for electrochemical cells or fuel cells by electrodepositing aluminum from an electrolyte containing an organo-aluminum complex compound upon a nickel or nickel alloy substrate, causing diffusion of some of the deposited aluminum into the nickel to form an alloy, from which aluminum is subsequently leached. The diffusion is carried out in an inert atmosphere at a temperature below 659.degree. C., preferably between 350.degree. and 650.degree. C. for about 1 or 2 hours. Very thin electrodeposited layers of 5 to 20 micron thickness are disclosed.
J. Yasamura and T. Yoshino, "Laminated Raney Nickel Catalysts", Ind. Eng. Chem. Prod. Res. Develop. 11, No. 3, p. 290-293 (1972), though not related to electrodes, discloses the preparation of Raney nickel plates by spraying molten aluminum onto a nickel plate, heating in a nitrogen atmosphere for an hour at 700.degree. C. to for form a 0.2-mm-thick layer of NiAl.sub.3 and then leaching aluminum from the layer. The resultant structure is reported to function effectively as a hydrogenation catalyst.
Another method of preparing shaped Raney nickel structures for use as hydrogenation catalysts is disclosed by Larson et al., U.S. Pat. No. 3,846,344. According to this patent, a nickel-coated metal tube is coated with an aluminum layer of at least 0.02-mm thickness; the aluminum is then diffused into the nickel by a heat treatment at a temperature of at least about 480.degree. C. for at least 30 minutes; and aluminum is then selectively dissolved from the diffused layer. Example 5 of the patent discloses subjecting a 25-mm-diameter tube, having a 1-mm-thick electroplated nickel layer on which a 0.5-mm-thick layer of aluminum was flame sprayed, to a diffusion heat treatment at 650.degree. C. for 6 hours to form an interdiffused layer of at least 0.05-mm thickness. The tube is then activated by immersion in a 25 percent solution of NaOH in water for 8 hours at 90.degree. C. The surface is reported to be highly effective for the catalytic hydrogenation of cyclohexane.
Dickinson et al. U.S. Pat. No. 3,407,231 discloses a method of making a negative electrode having an active porous nickel surface, for use in alkaline batteries. The patent discloses forming the electrode by bringing aluminum into contact with the surface of a nickel-containing core at an elevated temperature, thereby interdiffusing the aluminum and nickel to form a layer of nickel aluminide (Ni.sub.2 Al.sub.3), and then dissolving out the diffused aluminum with a caustic solution to produce a layer of active nickel metallurgically bonded to the core. Interdiffusion temperatures of 625.degree. to 900.degree. C. for 8 to 16 hours, dissolution temperatures of 20.degree. to 100.degree. C. for 1 to 32 hours, and coating thicknesses of 0.008 to 0.012 inch (200 to 300 microns) are mentioned. In particular, the process is described as being carried out by placing a nickel sheet in a pack consisting of a mixture of approximately 58% Al.sub.2 O.sub.3, 40% Al powder and 2% NH.sub.4 Cl; heating the pack in a reducing atmosphere for 8 hours at 800.degree. C. to form a layer of Ni.sub.2 Al.sub.3 of 0.008-inch (200 microns) thickness on each face of the nickel plate; and then immersing the coated nickel core in a solution of 6 normal NaOH for about 16 hours at 80.degree. C. to dissolve at least 85% of the aluminum out of the structure. However, we have found the Raney nickel surfaces of electrodes prepared by this particular process to be of low porosity. In lieu of the disclosed pack method for forming the Ni.sub.2 Al.sub.3 layer, the patent suggests rolling the nickel sheet between two aluminum sheets to form a metallic bond and heating the layered structure in a reducing atmosphere at a temperature of 1100.degree. F. (543.degree. C.). Although temperatures below 1200.degree. F. (649.degree. C.) are preferred in this alternative method, temperatures as high as 1600.degree. F. (872.degree. C.) are suggested. However, we have found that roll bonding does not produce the desired metallic bond.
Although the art has suggested various methods for making porous nickel structures and has reported low hydrogen overvoltage characteristics for some of them, none of these structures, to applicant's knowledge, has been used as an electrode in a large-scale commercial cell for the electrolysis of brine into chlorine, hydrogen and caustic. Perhaps this resulted from the problems of spalling, mechanical weakness and insufficiently low hydrogen-overvoltage characteristics of the prior art structures as well as from technical and economic problems associated with their manufacture. To overcome, or at least minimize, these problems, applicant has now produced an unusually active porous-nickel-coated electrode of the type that can be used advantageously as the cathode of a large-scale chlorine-alkali cell.